1. Field of the Invention
The present invention relates generally to a hybrid transmission suitable for a hybrid vehicle in which a prime mover such as an engine and at least one motor/generator (a motor functions as a generator or vice versa) are mounted. The present invention relates particularly to a gear shift control apparatus for the hybrid transmission which is capable of making a differential unit which couples between the prime mover (engine) and motor/generator(s) carry out a continuously variably transmitting operation using at least two differential units coupling between the prime mover and the motor/generator.
2. Description of the Related Art
A hybrid transmission is generally of a series-connection type, a parallel-connection type, and a combination type of series-and-parallel-connection. In any type of the hybrid transmissions, a whole or part of engine revolution energy is, once, converted into an electrical energy by means of a generator. It is general practice that powers of this electrical energy and from a power battery cause a motor coupled to a vehicular drive system to be driven to run the vehicle and an extra electrical energy is charged into the battery. An engine operating point is defined to achieve an optimum fuel economy. A charge and a discharge to and from the battery are carried out at a favorable timing so that a driving force in accordance with a driving condition can be developed under a favorable fuel consumption. A general hybrid transmission has a large passing power of the motor/generator in a low side transmission gear (or shift) ratio region including a reverse gear (or shift) ratio. The large passing power requires a large sized (large capacity) motor/generator and a width of the gear (shift) ratio which is capable of transmitting an engine power becomes narrower.
A Japanese Patent Application First Publication No. 2000-62483 published on Feb. 29, 2000 (which corresponds to a U.S. Pat. No. 5,931,757 issued on Aug. 3, 1999) exemplifies a previously proposed hybrid transmission (two-mode compound-split electro-mechanical vehicular transmission). In the above-described Japanese Patent Application First Publication, the gear (shift) region is divided into two regions, viz., a low side gear (or shift) ratio region including a reverse (reverse run) gear ratio and a high-side gear ratio region. An alternative between a clutch and a brake causes a switch between gear shift (gear ratio) modes. Even in the low-side gear ratio region including the backward (reverse) gear ratio, the alternative type hybrid transmission which has aimed at preventing the passing power through motor/generator from becoming large has been proposed.